1. Field of the Invention
The present invention relates to a method for operating a temperature-limiting device of an electrical unit activatable for a limited time period by way of an activation signal. The invention further relates to a temperature-limiting device, and to an electrical unit.
2. Description of the Art
Methods of the kind recited above are known from the existing art. The temperature-limiting device is usually associated with the electrical unit. The temperature-limiting device serves to permit activation of the electrical unit upon occurrence of the activation signal only when the temperature of the electrical unit, or an estimated temperature of the electrical unit, is below a specific temperature, in particular a limit operating temperature of the electrical unit. This means that the temperature-limiting device serves to limit the temperature of the electrical unit in such a way that neither temperature-related damage to the electrical unit nor a reduction in its service life can occur. The electrical unit is, for example, operated in such a way that it is at first deactivated, upon occurrence of the activation signal is activated only briefly, i.e. over the limited time period, and is then deactivated again. The electrical unit can be, in particular, a starter device of a motor vehicle, i.e. serves to start an internal combustion engine of the motor vehicle. Upon occurrence of the activation signal, the electrical unit or starter device is therefore operated so as to bring the internal combustion engine to a predetermined rotation speed and thus start it. The electrical unit is then deactivated again and, for example, operated as a generator. While the electrical unit is activated, it generates heat, which raises its temperature. If the electrical unit is activated over too long a time period, or is activated too often within a specific time span, the temperature of the electrical unit can then become greater than the limit operating temperature. As already stated above, this can result in damage to the electrical unit.
Published German patent application document DE 199 46 808 A1, for example, discloses an electrical starter apparatus for an internal combustion engine. The electrical starter apparatus possesses a thermal monitoring protection system for taking the starter apparatus out of service when its limit operating temperature is reached. The starter apparatus has a device for identifying a virtual operating temperature, in which context the device identifies the virtual operating temperature as a function of at least one operating parameter influencing the operating temperature of the starter apparatus. The virtual operating temperature is identified, for example, as a function of a current ratio that relates an actual starter current to a reference current. The reference current is, in particular, a rated current of the starter apparatus. The virtual operating temperature is then calculated, from the current ratio by integration. In order to determine the virtual operating temperature as accurately as possible, the integration must occur with a short time interval. Starter apparatuses of this kind are therefore constantly carrying out relatively complex calculations in order to determine the virtual operating temperature of the starter apparatus, and to take it out of service when its limit operating temperature is reached. The calculation unit needed to execute these calculations is, however, expensive.